Page 377 - Handbook of Energy Engineering Calculations
P. 377

environmental  regulatory  authorities,  and  (2)  to  recharge  the  geothermal
               resource.
                  One recent trend in the industry is to collect noncondensable gases (NCGs)
               purged  from  the  condenser  and  reinject  them  along  with  the  brine.  Older
               plants use pollution-abatement devices to treat NCGs, then release them to

               the atmosphere. Reinjection of NCGs with brine lowers operating costs and
               reduces gaseous emissions to near zero.
                  Major improvements in flashed-steam plants over the past decade centered

               around  are:  (1)  improving  efficiency  through  a  dual-flash  process  and  (2)
               developing improved water treatment processes to control scaling caused by
               brines. The pressure of the liquid brine stream remaining after the first flash
               is further reduced in a secondary chamber to generate more steam. This two-
               stage  process  can  generate  20  to  30  percent  more  power  than  single-flash

               systems.
                  Most of the recent improvements in binary-cycle plants have been made by
               applying new working fluids. The thermodynamic and transport properties of

               these fluids can improve cycle efficiency and reduce the size and cost of heat-
               transfer equipment.
                  To illustrate: By using ammonia rather than the more common isobutane or
               isopentane, capital cost can be reduced by 20 to 30 percent. It is also possible
               to  improve  the  conversion  efficiency  by  using  mixtures  of  working  fluids,

               which in turn reduces the required brine flow rate for a given power output.
                  A  flashed-steam  cycle  will  be  tentatively  chosen  for  this  installation
               because the brine free-flows at 450°F (232°C), which is higher than the cutoff

               temperature of 350°F (177°C) for binary systems. An actual plant (Fig.  5),
               operating  with  these  parameters  uses  two  flashes.  The  first  flash  produces
                                                                             2
               623,000 lb/h (283,182 kg/h) of steam at 100 lb/in  (gage) (689 kPa). In the
               second flash an additional 262,000 lb/h (117,900 kg/h) of steam at 10 lb/in                  2
               (gage) (68.9 kPa) is produced.
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